1. Field of the Invention
This invention relates to improvements in water-based well servicing fluids which better enable drilling and well servicing at high temperatures and high pressures. More particularly, this invention relates to polymer additives for water-based drilling fluids and other wellbore servicing fluids that afford desirable filtration control and rheological properties after exposure to a prescribed HTHP aging process.
2. Description of Relevant Art
Water based or aqueous based drilling fluids or muds are well known in the oil and gas industry, as are certain synthetically prepared polymers belonging to the vinyl sulfonate/vinyl amide (VSVA) class of polymers for use in such fluids. The polymer compositions often include the vinyl sulfonate monomer 2-acrylamide-2-methyl-1-propanesulfonic acid (AMPS). The vinyl amide structure of the polymer is often derived from the monomer acrylamide (AM).
Several AMPS/AM (or VSVA) polymers are available commercially for use as high temperature/high pressure (HTHP) filtration control additives for water based muds. Examples of such polymers include KEMSEAL™, available from Baker Hughes, Inc. in Houston, Tex., DURALON™, available from MI, L.L.C. in Houston, Tex., DRISCAL®-D, available from Phillips Petroleum Co. in Bartlesville, Okla., and ALCOMER® 242, available from Allied Colloids Ltd of the United Kingdom. Such commercially available polymers typically comprise two monomers AMPS and AM and are either linear or crosslinked in structure. During preparation, crosslinking of the VSVAs can be induced or controlled by the choice of reaction initiator chemistry or by the deliberate incorporation of some monomer known to induce crosslinking, such as methylene-bis-acrylamide or divinylbenzene. When VSVAs incorporate the monomer N-methyl-N-vinylacetamide (VIMA or VMA), the fluid typically shows desirable wellbore fluid performance characteristics under HTHP conditions. Random terpolymers consisting of AMPS, N-methyl-N-vinylacetamide (VIMA or VMA), and AM are commercially available and known to usually impart desirable performance characteristics to a water-based mud under HTHP conditions. Preferred examples are THERMA-CHEK™, available from Halliburton Energy Services, Inc., and HOSTADRILL® V2825, available from Clariant of Switzerland and previously from Hoechst Aktiengesellschaft of Germany.
Although VSVA polymers are known to typically impart desirable mud performance characteristics under HTHP conditions to water-based muds, and such polymers have been available for many years, there is a need for improved fluid performance beyond that which the VSVA polymers can provide.